Summary of Significant Accounting Policies - Business, Fair Value, and Goodwill (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item	Dec. 31, 2013 USD ($)
Summary of Significant Accounting Policies
Minimum number of locations in which company operates metal service network | item	300
Number of states in which the company operates metal service center network | item	39
Number of countries in which entity operates | item	12
Minimum number of metal products distributes by the company | item	100,000
Fair Values of Financial Instruments
Carrying value, before deducting unamortized discount or premiums | $	$ 1,942.1	$ 2,319.1
Carrying value | $	$ 1,942.1
Goodwill Policy
Number of operating segments | item	1
Number of reportable segments | item	1	1
Impairment of goodwill | $	$ 0.0	$ 0.0	$ 0.0
Senior Unsecured Notes
Fair Values of Financial Instruments
Carrying value, before deducting unamortized discount or premiums | $	1,100.0
Fair value | $	1,080.0	1,160.0
Carrying value | $	$ 1,090.0	$ 1,090.0